The present invention relates to improvements in signal-processing systems and more particularly to improved techniques for eliminating interference introduced into the mainlobe of an antenna from an interference source.
Signal-processing equipment in general is designed with a goal of receiving only particular information for evaluation. However, as is often the case, desired information is not isolated by itself but may be found in the presence of unwanted signals. Antenna systems in particular have characteristics that include a mainlobe for receiving desired information and a plurality of sidelobes at various angles relative to the mainlobe. Due to the nature of an antenna, information received in a sidelobe is indistinguishable from information received in the mainlobe and thus renders the equipment highly susceptible to interference from unwanted signals or information. This problem is particularly acute in radar systems where the presence of sidelobes makes it possible for a single noise jammer to be effective against a radar from any angle of azimuth.
Sidelobe cancellation is a fundamental approach to eliminating interference in received signals and has been used relatively successfully to eliminate the interference introduced from a single jamming source. Generally, to provide successful cancellation, the sidelobe canceller employs auxiliary omni-directional antennas, receiving channels and adaptive cancellation loops to remove interference signals which enter the sidelobe response of a radar system. The adaptive loops function by adjusting the phase and amplitude of the received auxiliary signals such that they subtract out the interference present in the main radar channel.
Such a system is taught, for example by U.S. Pat. No. 3,938,154 issued to Bernard L. Lewis on Aug. 16, 1964. The gains of the auxiliary channels are nominally made much less than the mainlobe gain of the radar system in order to prevent cancellation of legitimate target-return signals. This relative gain difference prohibits the sidelobe canceller from effectively cancelling direct-path jamming interference received by the radar mainlobe.
Typical performance of a conventional coherent sidelobe cancellation system against a pair of barrage-jamming sources is illustrated by the plan position indicator photos shown in FIGS. 1 and 2. The photo shown in FIG. 1 corresponds to no cancellation, while the photo shown in FIG. 2 was taken with cancellation. Note the presence of the two mainlobe jamming residues which remain in the cancelled case.
Two primary methods have been proposed to remove or reduce this mainlobe jamming residue. The first method depends on utilizing a directional antenna and receiving system with a gain approximately equal to that of the main radar channel but with a directional response which is offset in azimuth from the peak of the mainlobe. This additional system then provides interference signals which are of sufficient magnitude to subtract out mainlobe interference. Some target-return signals common to both systems will also be subtracted out and the net result is a relative narrowing of the mainlobe. The practical problem of providing an additional receiving system equivalent to the main radar system is a drawback of this technique.
The second mainlobe interence technique is simpler a gain control function. The mainlobe jamming residue is used with an adaptive loop to cancel itself (with cancellation being directly proportional to received jamming power). Since all signals are reduced by the same amount in this techniques, the result is largely cosmetic with no improvement in the target signal-to-jamming interference ratio of the main channel.